bug#58797: 29.0.50; Revise format of stored message tags in ERC

[J.P.]

Tags: patch

I'm proposing we change the format (type) of the "tags" field in the
`erc-response' struct from

  (STRING . LIST)

where LIST contains at most one (possibly empty) string, to

  (SYMBOL . OPT-STRING)

where OPT-STRING, when non-nil, is a nonempty string. For ERC 5.5 and
Emacs 29, this change would merely manifest as a warning when first
encountered. In subsequent releases, it would still be reserved only for
users who opt in by activating optional modules. Rationale below.

Thanks,
J.P.


Imagined FAQ:

  Why bother?

    In the existing implementation, all code accessing a processed tag
    value must check that it's both non-nil and nonempty. This stands to
    become a common occurrence with the arrival of IRCv3 in future
    releases. The majority of instances will only contain one item (a
    timestamp) and will persist beyond the parsing stage as text
    properties in ERC buffers. And since we need to redo the parsing
    anyway to abide by all the escaping rules and other spec minutiae,
    we might as well make the processed goods as easy to manage as
    possible, no?

  Isn't this a breaking change?

    Yes. But I believe preserving backward compatibility (by adding an
    additional field and keeping the old one around) is less important
    in this case because the field currently goes unused in ERC's client
    code and is likely underused in user code (based on a cursory survey
    of Melpa packages, popular shared configs, and wiki snippets). While
    message tags *are* currently parsed when present, the field is only
    ever written to and never again accessed (on account of ERC's lack
    of general IRCv3 support). That said, the library function as well
    as the `erc-response' structure are indeed public (exported).

    There is also the matter of authoritative intent to consider. The
    doc string for the function `erc-parse-tags' describes its return
    type as being an alist of "(tag . value)". For anyone willing to
    entertain "value" as meaning "logical tag value," this change
    becomes a belated bug fix.

  Why now? Why not wait?

    It's true that the main benefit of doing this won't be felt for a
    couple more ERC releases. As stated above, the immediate effect of
    this change will merely be as a warning for any user code that
    depends on this field. This approach seems somewhat in keeping with
    the overall Emacs policy for function and variable deprecation.


In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.34, cairo version 1.17.6) of 2022-10-26 built on localhost
Repository revision: 832bd2dfe51b2b9ea5e0e56e0e6ee60b2d8ae2ed
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 36 (Workstation Edition)

Configured using:
 'configure --enable-check-lisp-object-type --enable-checking=yes,glyphs
 'CFLAGS=-O0 -g3'
 PKG_CONFIG_PATH=:/usr/lib64/pkgconfig:/usr/share/pkgconfig'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS WEBP X11 XDBE XIM XINPUT2 XPM GTK3 ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search time-date subr-x mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
cl-loaddefs cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util
mail-prsvr mail-utils rmc iso-transl tooltip cconv eldoc paren electric
uniquify ediff-hook vc-hooks lisp-float-type elisp-mode mwheel
term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit xinput2 x multi-tty make-network-process
emacs)

Memory information:
((conses 16 36089 6500)
 (symbols 48 5118 0)
 (strings 32 13124 1756)
 (string-bytes 1 372990)
 (vectors 16 9304)
 (vector-slots 8 147959 9387)
 (floats 8 21 29)
 (intervals 56 220 0)
 (buffers 1000 10))

0001-Warn-of-future-breaking-change-to-erc-response.tags.patch
Description: Text Data